Performance of tungsten fibers for Wf/W composites under cyclic tensile load

Toughness improved tungsten-based composites are one of the currently considered material option for future fusion reactors capable to withstand both high heat flux and irradiation induced embrittlement. Today, fiber-reinforced composites (Wf/W) are being intensively studied as risk-mitigation materials to replace bulk tungsten which is susceptible to neutron irradiation embrittlement especially below 800 °C. Operation of a material as an element of a plasma facing component (i.e. divertor monoblock or first wall armour) implies not only high heat flux exposure but also thermal cyclic fatigue caused by repetitive oscillations of the heat loads due to the nature of the plasma and the limitations on the capacity of its confinement. In this work, we assessed the performance of potassium doped tungsten fibers under cyclic loading applied in tensile mode. Stress-controlled fatigue tests were performed at room temperature, 300 °C and 500 °C increasing the load from 50% of the yield strength up to the ultimate tensile strength of the studied fibers. It is revealed that significant cyclic hardening emerges as the fatigue stress limit exceeds the yield strength already within a few cycles. Despite the noticed cyclic hardening, the wire can sustain few hundreds of cycles without any detectable damage unless the cycle stress is increased to reach the value above the mean ultimate tensile strength. Given this observation, we have studied the impact of the cyclic stress (σC) on the rupture strength and total elongation of the wires exposed to twenty loading cycles varying test temperature in the range 23–500 °C. At room temperature, the rupture stress after cyclic deformation progressively increases with σC and saturates at 2.7 GPa with a moderate reduction of the total elongation, while the nominal ultimate tensile strength of the wire is 2.5 GPa. Thus, the strength of the wire is increased by 200 MPa, on average. At elevated temperature, the rupture stress after the cyclic deformation increases by more than 300 MPa.